Preserve custom prefixes when expanding compound arguments

[fhgffy]

Compound arguments recognize configured prefix characters but expand each short option using a hard-coded -. With set_prefix_chars("+"), +ab therefore fails to find +a and +b. If both - and + options are registered, it can silently activate the wrong options instead.

Preserve the compound token's actual prefix in both parse_args and parse_known_args. Add regression coverage for default, plus and slash prefixes, value consumption, cross-prefix isolation, unknown arguments, and exact-name precedence.

Validation on master 4dbc0ffd3d5f83941eab12e73966724d8931ae07, GCC 14.2/C++17:

• Three regression cases fail against the original header
• Full suite: 278 cases and 1,022 assertions pass; one existing version test remains explicitly skipped
• Focused ASan/UBSan: seven cases and 49 assertions pass
• Independent review: 41 additional scenarios pass, including argc/argv, subparsers, multi-value compounds, missing values, and disabled prefixes
• git diff --check passes

Leak detection was disabled because LeakSanitizer cannot run under this environment's ptrace. Clang/MSVC and clang-tidy were not run locally.